It is known to make soft magnetic cores for electromagnetic devices such as transformers, inductors, motors, generators, relays, and the like, by pressing powdered iron into the desired core shape. The term "iron" as used herein applies not only to substantially pure iron but to the well known alloys thereof used for such purposes including, for example, Fe-Si, Fe-Al, Fe-Si-Al, Fe-Ni, Fe-Co, etc. Alloyed iron provides higher magnetic permeability and lower total core losses (i.e., eddy current, hysteresis and anomalous losses) and results in devices having higher efficiencies than devices using pure iron cores. It is likewise known that to insure that cores formed from such powders have low total core losses, the individual particles must be electrically insulated one from the other. On the other hand, to provide the maximum magnetic permeability the amount of interparticle insulation should be minimized and iron content maximized. Hence, cores made from polymer-bonded iron particles should have as low a polymer content as is possible which unfortunately tends to reduce the physical strength of the core. One known technique for electrically insulating the several particles from each other is to coat the surface of the particles with inorganic insulating materials such as iron phosphate or alkali metal silicate inorganic coatings, and/or organic polymeric materials such as: amber (Schulze U.S. Pat. No. 2,162,273); phenol-aldehyde condensation products (Roseby U.S. Pat. No. 1,789,477 or Hubbard U.S. Pat. No. 3,451,934); varnishes formed from China-wood oil and/or phenol resin (Polydoroff U.S. Pat. No. 1,982,689); resinous condensation products of urea or thiourea or derivatives thereof with formaldehyde (Eisenman U.S. Pat. No. 1,783,561); polymerized ethylene, styrene, butadiene, vinyl acetate, acrylic acid esters and derivatives thereof, copolymers of two or more of the foregoing as well as fluorine type polymers (Ochiai U.S. Pat. No. 4,696,725); radical polymerizable monomers such as styrene, vinyl acetate, vinyl chloride, acrylonitrile, acrylic acid esters, methacrylic acid esters, acrylic acid salts, methacrylic acid salts, divinyl benzene, N-methylol acrylamide and the like (Yamaguche U.S. Pat. No. 3,935,340); and silicones, polyimides, fluorocarbons and acrylics (Soileau et al U.S. Pat. No. 4,601,765). In some instances, the iron particles have an inorganic undercoating and an organic topcoat (e.g., Soileau et al supra, Katz U.S. Pat. No. 2,783,208 and VerWeij U.S. Pat. No. 3,232,352).
It has heretofore been proposed to polymer coat magnetic core-forming iron particles in a number of ways including: (1) dispersing the particles in a solution of the polymer dissolved in a solvent and driving off the solvent; (2) polymerizing the polymer in situ on the surface of the particles; and (3) coating the particles in a fluidized bed thereof with the polymer dissolved in an appropriate solvent.
While the aforesaid polymer-coated particles are capable of forming cores for some applications, none are seen to be satisfactory for readily compression or injection molding magnetic cores which have high permeability, low total core losses, high physical strength and are capable of surviving in chemically and thermally hostile environments such as are found in the engine compartment of an automobile where the core is often subjected to temperatures above about 200.degree. C., and a variety of corrodents including high humidity, salt, and fuel/lubricant vapors. Unfortunately, the more common polymers that one might expect would survive, and accordingly be useful in, such a hostile environment do not have the processability characteristics needed to completely coat the particles and/or to readily mold high density, high strength cores therefrom having the desired physical and magnetic properties.
Indeed most polymers otherwise suitable for such a hostile environment are thermosets which after having been once cured about the iron particle cannot be dissolved, reprocessed or compression/injection molded. On the other hand, most thermoplastics which might be both moldable and capable of withstanding the hostile environment cannot practically be coated uniformly and continuously onto small iron particles primarily because they are either essentially insoluble in industrially acceptable solvents (for example, crystalline thermoplastics), do not coat the particles well, cannot be readily handled in a heated condition preparatory to molding (e.g., becomes tacky), and/or have too high a melt viscosity for proper filling out of the shaping die during molding. On the other hand and as a general rule, amorphous thermoplastics would not be expected to survive the hostile environments owing to their solvent vulnerability in fuel and lubricant vapors and poor temperature resistance.
An ideal polymer would be a thermoplastic which can survive in a chemically and thermally hostile environment, which is soluble in industrially acceptable solvents for coatability, which serves as a lubricant for optimum densification of the particles under compression molding conditions, which has a low melt viscosity for optimal in-the-die flow when molten and which has a non-sticky surface at temperatures within about 110.degree. C. of its softening point for premolding handling and processability in a heated condition. In this latter regard, a non-sticky surface at this elevated temperature allows the particles to remain free-flowing at temperatures near the softening point which permits preheating them while still allowing automatic mechanical feeding of same into a heated die. This, in turn, results in shorter die cycle times and significantly stronger molded cores owing to a more uniform temperature throughout the particle mass in the die during molding. In this regard, the term softening point is intended to mean the temperature where the polymer becomes sufficiently fluid as to flow readily within the tooling (i.e., under pressures of about 20-50 TSI) to fill the die completely yet not be so "watery" as to separate from the particles. Cooler particles tend not to heat adequately in the center of the molded core resulting in a well fused shell surrounding a weaker fused center.